The present invention relates generally to a method and apparatus for cardiac pacing and, in particular, to a pacing system providing adjustable atrio-ventricular time delays to improve different heart performance parameters.
The heart is the center of the circulatory system. It is an organ which performs two major pumping functions and may be divided into right and left heart xe2x80x9cpumps.xe2x80x9d The left heart pump draws oxygenated blood from the lungs and pumps it to the organs of the body. The right heart pump draws blood from the body organs and pumps it into the lungs. For a human heart, the right heart pump is on a patient""s right side and the left heart pump is on the patient""s left side. Figures in this document, such as FIG. 1, show a xe2x80x9ctopxe2x80x9d view of the heart, which is the view that a physician observes during open heart surgery. Therefore, the left heart pump is on the right hand side of the FIG. 1 and the right heart pump is on the left hand side of FIG. 1. Each heart pump includes an upper chamber called an atrium and a lower chamber called a ventricle. The left heart pump therefore contains a left atrium (LA) and a left ventricle (LV), separated by a valve called the mitral valve. The right heart pump contains a right atrium (RA) and a right ventricle (RV), separated by a valve called the tricuspid valve.
The blood flows in the circulatory system in the following path: from the peripheral venous system (blood which has transferred through the body organs) to the RA, from the RA to the RV through the tricuspid valve, from RV to the pulmonary artery through the pulmonary valve, to the lungs. Oxygenated blood from the lungs is drawn from the pulmonary vein to the LA, from the LA to the LV through the mitral valve, and finally, from the LV to the peripheral arterial system (transferring blood to the organs of the body) through the aortic valve.
Normally, the heart pumps operate in synchrony and ensure the proper pumping action to provide oxygenated blood from the lungs to the organs of the body. A normal heart provides this synchrony by a complex conduction system which propagates electrical pulses to the heart muscle tissue to perform the necessary atrial and ventricular contractions. A heartbeat is the result of a regular train of electrical pulses to the proper portions of the heart to provide rhythmic heart pumping. The heart muscle provides pumping by the contraction of muscle tissue upon receipt of an electrical signal, and the pumping action is made possible through a system of heart valves which enable blood flow in a single direction. Thus, the heart includes a complex electrical and mechanical network.
To pump blood through the circulatory system, a beating heart performs a cardiac cycle. A cardiac cycle consists of a systolic phase and a diastolic phase. During systole, the ventricular muscle cells contract to pump blood through both the pulmonary circulation and the systemic circulation. During diastole, the ventricular muscle cells relax, which causes pressure in the ventricles to fall below that in the atria, and the ventricles begin to be refilled with blood.
In normal condition, the cardiac pumping is highly efficient. One aspect of this high efficiency is due to sequential atrio-ventricular contraction. Near the end of diastole, the atria contract, causing an extra amount of blood to be forced into the ventricles. Thus, the ventricles have more blood (preload) to pump out during next systole. Another aspect of this high efficiency in blood pumping is contributed from a network or fast ventricular conduction system. As shown in FIG. 1, the system includes right and left bundle branches of conductive tissues that extend from the Bundle of His and the massive network of fast conducting Purkinje fibers that cover most of the endocardial surface of the ventricles. Electrical signals coming from the atrium are relayed to the Purkinje fibers through the bundle branches, and to the different regions of the ventricles by the Purkinje fiber network. Therefore the entire ventricular muscle cells can contract synchronously during systole. This synchronized contraction enhances the strength of the pumping power.
To assess the cardiac function, it is important to examine the LV systolic performance which directly determines the ability of the heart to pump blood through the systemic circulation. There are multiple ways to assess the performance of the heart. One way is to examine how well the LV contracts in order to determine the effectiveness of the LV as a pump. As can be seen from FIG. 2, the LV starts to contract after an electrical signal propagating down the left bundle branches stimulates muscle cells of septal wall M and lateral wall N. In FIG. 3, the walls M and N are contracting such that they are forced towards each other to pump blood out of the ventricle. One measure of LV contraction effectiveness is called xe2x80x9ccontractility.xe2x80x9d Left ventricular contractility is a measure of overall strength of the contracting power of the LV muscle cells. It is a function of the health of the LV muscle tissue and the coordination of the contractions of the entire LV, including walls M and N. Such coordination depends on the health of the left bundle branches and on the health of the fast conducting Purkinje fiber network. LV contractility is estimated by measuring the peak positive rate of change of the LV pressure during systole. In mathematical terms, this is the maximum positive derivative of the LV pressure, which is denoted by the term xe2x80x9cLV+dp/dfxe2x80x9d.
LV systolic performance is also measured by stroke volume, which is the volume of blood pumped out of the LV per systole. Stroke volume can be estimated by measuring aortic pulse pressure (PP).
Cardiac muscle cells need to be electrically excited before they can have a mechanical contraction. During the excitation (depolarization), electrical signals will be generated and they can be recorded both intracardially and extracardially. The recorded signals are generally called electrocardiogram (ECG). An ECG recorded intracardially is also called an electrogram, which is recorded from an electrode placed endocardially or epicardially in an atrium or a ventricle. An ECG recorded extracardially is often called surface ECG, because it is usually recorded from two or more electrodes attached to the skin of the body. A complete surface ECG recording is from 12-lead configuration.
The features in ECG are labeled according to the origin of the electrical activity. The signals corresponding to intrinsic depolarization in an atrium and a ventricle are called P-wave and QRS complex, respectively. The QRS complex itself consists of a Q-wave, a R-wave, and a S-wave. The time interval from P-wave to R-wave is called PR interval. It is a measure of the delay between the electrical excitation in the atrium and in the ventricle.
Several disorders of the heart have been studied which prevent the heart from operating normally. One such disorder is from degeneration of the LV conduction system, which blocks the propagation of electric signals through some or all of the fast conducting Purkinje fiber network. Portions of the LV that do not receive exciting signals through the fast conducting Purkinje fiber network can only be excited through muscle tissue conduction, which is slow and in sequential manner. As a result, the contraction of these portions of the LV occurs in stages, rather than synchronously. For example, if the wall N is affected by the conduction disorder, then it contracts later than the wall M which is activated through normal conduction. Such asynchronous contraction of the LV walls degrades the contractility (pumping power) of the LV and reduces the LV+dp/dt (maximum positive derivative of the LV pressure) as well.
Another disorder of the heart is when blood in the LV flows back into the LA, resulting in reduced stroke volume and cardiac output. This disorder is called mitral regurgitation and can be caused by an insufficiency of the mitral valve, a dialated heart chamber, or an abnormal relationship between LV pressure and LA pressure. The amount of the back flow is a complex function of the condition of the mitral valve, the pressure in the LV and in the LA, and the rate of blood flow through the left heart pump.
These disorders may be found separately or in combination in patients. For example, both disorders are found in patients exhibiting congestive heart failure (CHF). Congestive heart failure (CHF) is a disorder of the cardiovascular system. Generally, CHF refers to a cardiovascular condition in which abnormal circulatory congestion exists as a result of heart failure. Circulatory congestion is a state in which there is an increase in blood volume in the heart but a decrease in the stroke volume. Reduced cardiac output can be due to several disorders, including mitral regurgitation (a back flow of blood from the LV to the LA) and intrinsic ventricular conduction disorder (asynchronous contraction of the ventricular muscle cells), which are the two common abnormalities among CHF patients.
Patients having cardiac disorders may receive benefits from cardiac pacing. For example, a pacing system may offer a pacing which improves LV contractility, (positive LV pressure change during systole), or stroke volume (aortic pulse pressure), however, known systems require complicated measurements and fail to provide automatic optimization of these cardiac performance parameters. Furthermore, the measurements are patient-specific and require substantial monitoring and calibration for operation. Therefore, there is a need in the art for a system which may be easily adapted for optimizing various cardiac parameters, including, but not limited to, LV contractility, (peak positive LV pressure change during systole, LV+dp/dt), and cardiac stroke volume (pulse pressure). The system should be easy to program and operate using straightforward patient-specific measurements.